This invention relates generally to nuclear magnetic resonance imaging (MRI), and more particularly the invention relates to motion analysis and imaging of an organ such as the heart using phase contrast MRI maps of tissue velocity in the organ.
Assessment of the motion of the heart muscle, myocardial motion, is fundamental to the characterization of certain cardiac pathologies and to the development and evaluation of successful interventions. Disclosed in U.S. Pat. No. 4,710,717 is a method of studying heart motion using magnetic resonance imaging (MRI). MRI is a noninvasiye method which provides measures of global myocardial function and full field anatomical images. Briefly, in the method according to the '717 patent, data are acquired at rapid rates and the incrementation of the amplitude of the phase encoding gradient is controlled using a physiological trigger, e.g. EKG. While this occurs, the temporal position within the cardiac cycle at which each echo was acquired is also measured. Using this timing information and interpolation methods, images that portray the appearance of the object throughout the cardiac cycle can be formed. While useful for cardiac studies, these images still suffer from a disadvantage it shares with most other noninvasive methods of cardiac imaging. Specifically, the method images the tissue that is present in a given physical plane throughout the cycle and not the actual motion of specific tissue samples. Especially as a result of the lack of contrast or features within the imaged structure, many important aspects of the motion cannot be assessed.
Methods that produce images whose intensity is proportional to velocity have also been demonstrated. See, for example, O'Donnell, Med. Physics, 12: 59-64, 1985; Spirtzer Radiology, 176: 255-262, 1990, and Nayler et al., J. Computer Assisted Tomography, 10: 715-722, 1986. These methods generally belong to the class of methods called phase contrast MRI. Co-pending U.S. patent application Ser. No. 07/564,945, filed Aug. 9, 1990, for Encoding For NMR Phase Contrast Flow Measurement, discloses a particularly useful and efficient method for simultaneously measuring the three components of velocity, as well as an apparatus with which the method can be performed. Phase contrast principles have been combined with the cine imaging method described above to enable the production of images that portray the distribution of velocities at multiple points in the cardiac cycle. See Pelc, et al., Magnetic Resonance Quarterly, Vol. 7, No. 4, October, 1991, pp. 229-254.
The recently introduced myocardial tagging method with MRI, however, offers a noninvasive technique for obtaining information about the motion of specific myocardial sites similar to that derived from implanted markers. See Zerhouni et al., "Human Heart: Tagging with MR Imaging--A method for Noninvasive Assessment of Myocardial Motion", Radiology 1988; 169:59-63, and Axel, "MR Imaging of Motion with Spatial Modulation of Magnetization", Radiology 1989; 171:841-845.
Co-pending application Ser. No. 07/617,904 filed Nov. 26, 1990, supra, discloses apparatus and method for analyzing the motion of specific regions using phase contrast cine data. A problem with phase contrast cine MRI relates to additive velocity errors which typically arise due to eddy currents induced by the magnetic field gradients. While many eddy current effects are common to the measurements used to derive velocity information, the gradient changes used to encode velocity can cause differential eddy current effects, and these in turn produce unwanted phase shifts in the images. These unwanted phase shifts appear as additive velocity errors in the images. For example, static structures appear to have small nonzero velocity. Because of the repetitive manner in which the NMR sequences are implemented and the way the cine acquisition is performed, these additive errors can be assumed to be constant throughout the cycle. Also, because of the nature of the magnetic fields that induce the eddy current, the unwanted phase shifts vary very slowly across the images. The additive velocity errors produce errors in the computed motion. The apparent motion between the first and second frame will be composed of a true motion plus a motion error proportional to the additive velocity error. To the extent that the velocity error is constant throughout the cycle and throughout the motion of the object, motion error will accumulate and grow throughout the cycle.